That gap is 4 percentage points narrower than a similar CNN poll of polls conducted March 26 through Wednesday. In that average, Clinton led the senator from Illinois 51 percent to 40 percent. Nine percent also were unsure then.

The most recent poll of polls consists of three surveys: American Research Group (April 5-6), Muhlenberg College (March 27-April 2) and Quinnipiac (March 24-31). A sampling error for the poll of polls can not be calculated.

A decisive win in Pennsylvania's April 22 primary vote is seen as crucial for Clinton as she seeks to finish the primary season with enough momentum to convince the party's undecided superdelegates to give her the nomination.

But according to University of Pennsylvania professor Donald Kettl, some of the "air [is] starting to slip out of Hillary Clinton's tires."

So what's behind the shift?

"Obama has outspent Hillary Clinton three to one just on television advertising in Pennsylvania. He spent more than $3 million dollars trying to get his name out and his message out to Hillary Clinton's $1 million dollars," said Mark Preston, a CNN political editor.

Political observers point out that polls are not votes, and there's still a lot of time for anything to happen.

"We've got two weeks left before the primary and the way this campaign has gone, that's an eternity, if you just look at what's happened just day by day, let alone week by week along the way," Kettl said.

And as the primary nears, two prominent Clinton supporters suggested last week that she needs to best Obama in the popular vote to have any chance at winning the Democratic nomination.

In separate interviews, New Jersey Gov. Jon Corzine and Pennsylvania Rep. Jack Murtha of Pennsylvania indicated they believed Clinton will be unable to convince enough superdelegates to support her if she finishes second to Obama in both the pledged delegate count and the popular vote.

Speaking Thursday on CNBC, Corzine suggested it won't be enough for Clinton to argue she deserves the nomination because she has won more crucial swing states than Obama -- a talking point her campaign has long argued.

Murtha echoed Corzine's sentiments in an interview later Thursday. "Clinton has to win Pennsylvania," Murtha said. "She has to be ahead in the popular vote to have any chance at all of getting this nomination."

According to the latest CNN count, Obama has 1,629 delegates, with 1,414 pledged delegates and 215 superdelegates, according to the latest CNN count.